Pleochroic dyes are roughly classified into two groups. One group dyes are those in which the direction of transition moment of visible light absorption is almost in parallel with the direction of the longer axis of the molecule and which, when dissolved as a guest molecule in a host liquid crystal, are aligned so that the direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having parallel dichroism (or P-type dyes). The other group dyes are those in which the direction of transition moment of visible light absorption is almost perpendicular to the direction of the longer axis of molecule and which, when dissolved as a guest molecule in a liquid crystal, are aligned so that the direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having perpendicular dichroism (or N-type dyes). The present invention relates to a liquid crystal composition containing a dye belonging to the first group, i.e., pleochroic dye having parallel dichroism.
Pleochroic dyes are characterized in that the degree of light absorption depends upon the relative relation between the direction of absorption transition moment of dye molecule and that of electric vector of light. That is, the absorption becomes maximum when the direction of absorption transition moment is in parallel with the electric vector of light, and becomes minimum when the direction of absorption transition moment is perpendicular to the electric vector of light.
When a nematic, cholesteric or smectic liquid crystal containing such pleochroic dye is sandwiched between facing two electrodes and a voltage is applied to it across the electrodes, the liquid crystal molecules exhibit turbulent motion or are uniformly aligned along the direction of electric field depending upon the dielectric characteristics or fluid characteristics of the liquid crystal. In this occasion, the pleochroic dye molecules also move together with the liquid crystal molecules, and hence relative relation between the direction of absorption transition moment of the pleochroic dye molecule and the electric vector of incident light is changed, and consequently, the light absorption characteristics of a liquid crystal display devices are changed.
Such a phenomenon is widely known as "guest-host effect" and a color display device using electric control can be constituted by utilizing this effect (see "Guest-Host Interaction in Nematic Liquid Crystals: A New Electro-Optic Effects" reported by G. H. Heilmeier and L. A. Zanoni in Applied Physics Letters, Vol. 13, p. 91 (1968)).
The pleochroic dyes to be used as a guest in a liquid crystal display utilizing the above-described guest-host effect are required to possess: (1) a high "order parameter" in a host liquid crystal; (2) a hue according to the end-use; (3) a sufficient solubility in a host liquid crystal; and (4) a high stability (light stability, heat stability, and electric stability).
Of the above-described requirements, order parameter (1) (usually presented as S) means the degree of alignment of absorption axis of dye molecule with respect to orientation direction of host liquid crystal molecules, and is defined by the following equation: EQU S=1/2(3 cos.sup.2 .theta.-1)
wherein the term of cos.sup.2 .theta. is timewise averaged, and .theta. represents an angle which the absorption axis of the dye molecule makes with the orientation direction of host liquid crystal molecules. The order parameter S of pleochroic dye molecule is experimentally determined by the following equation: ##EQU1## wherein A.sub..parallel. and A.perp. represent the absorbances of the dye molecules for the light polarized parallel to and perpendicular to the orientation direction of the host guest crystal, respectively.
Specifically, the order parameter S is a value which governs the contrast of a guest-host type liquid crystal display device. With pleochroic dyes having parallel dichroism, the nearer the value to 1 which is the theoretical maximum, the less the degree of residual color in white background parts, which serves to realize bright and highly contrasty display.
Additionally, the ratio of A.sub..parallel. to A.perp., A.sub..parallel. /A.perp., is called dichroism ratio (usually presented as R), and represents the degree of parallelism of the absorption axis of dye molecule with respect to the direction to which host liquid crystal molecules are oriented, similarly with the order parameter. With pleochroic dyes having parallel dichroism, the dichroism ratio R takes a value of 1 or more and, the more the value, the brighter and more contrasty becomes the display.
The value of order parameter S required for a pleochroic dye is difficultly specified, because it depends upon the end-use and the conditions of use of the dye-containing guest-host type element but, usually, it is said to be desirably at least 0.5, preferably 0.7 or more, at about room temperature.
As to the hue (2) referred to hereinbefore, the dyes must satisfy requirements for a wide variety of hues, taking into consideration the purposes of coloration such as to increase displayed information, increase degree of freedom of design, and improve fashionability. Basically, if three primary colors of yellow, magenta, and cyan blue are obtained, all hues can be obtained by subtractive mixture of them. Therefore, with respect to the problem of hue, it is of importance to obtain a pleochroic dye showing one of the three primary colors.
As to the solubility (3), fundamentally the higher, the better. It is needless to say that minimum solubility required varies depending upon the end-use and the condition of use of a guest-host element containing the pleochroic dye.
An object of the present invention is to provide a liquid crystal composition containing a pleochroic dye which satisfies the fundamental requirements (1), (2) and (4) described hereinbefore, which shows a yellow color, one of the primary colors, and which gives a strong fluorescence.
It is well known that a yellow color is important not only as one of the primary colors but as an ingredient for preparing a practically useful black or green color.
In addition, in a guest-host type liquid crystal display, the use of a pleochroic dye having strong fluorescent properties as a guest provides the possibility of improving visibility and design properties of the liquid crystal element, which serves to expand the application of the element.
Further, the use of a pleochroic dye having strong fluorescent properties is advantageous in that it enables to constitute a special structure guest-host type display utilizing the strong fluorescence as well as an ordinary guest-host type display. (Examples of such guest-host type liquid crystal display utilizing fluorescence are found in A. Hochbaum, L. J. Yu and M. M. Labes, Journal of Applied Physics, Vol. 51, p. 867 (titled as "Fluorescence of guest molecules in a scattering state of a liquid crystal" (1980), Japanese Patent Laid-Open No. 48571/79, etc.)
Relationship between the molecular structure of pleochroic dye and the various properties has not fully been clarified yet, and hence it is quite difficult to select a pleochroic dye which has a desired hue and satisfies all requirements described hereinbefore based on knowledges about known dyes.
As yellow pleochroic dyes, there have heretofore been known, for example, azo or azomethine type dyes as follows: ##STR2## (See H. Seki et al., Preliminary Drafts for the 27th Applied Physics Combined Lecture Meeting, 114 (1980), titled "Guest-Host Effect of Liquid Crystals".)
However, fluorescence of these pleochroic dyes is too weak to be clearly recognized visually. With azomethine dyes, the bond of --N.dbd.CH-- in the molecule is known to react with water to cause hydrolysis reaction as follows: EQU --NH.dbd.CH--+H.sub.2 O.fwdarw.--NH.sub.2 +OHC--
As the sealing material for at present practically used liquid crystal display elements, organic sealing materials are primarily used. In general, such organic sealing materials are so water-permeable that it is quite possible that water having permeated into the liquid crystal reacts with the azomethine dye as illustrated above to adversely affect various properties of the element.
With the above-described prior art in mind, the inventors have made intensive investigations and, as a result, the inventors have found the structure of perylene dyes which show a yellow color, which have strongly fluorescent properties, and which possess excellent order parameter, excellent solubility, and excellent stability, thus having achieved the present invention based on the findings.
As the examples of using perylene dyes in liquid crystal display prior to the present invention, the following dye is seen in Japanese Patent Laid-Open No. 48571/79, etc.: ##STR3##
When this dye was actually synthesized and dissolved in a phenylcyclohexane type mixed liquid crystal of ZLI-1132 to be described hereinafter to measure the order parameter according to the method to be described hereinafter, the order parameter of the dye was found to be only 0.46, which is less than the usually desired level.
Thus, the inventors have examined the kind, number, and position of substituents to be introduced to the perylene skeleton with many compounds. The perylene skeleton is numbered as follows, and selection of kind, number, and position of substituents on the perylene skeleton exerts great influences on the characteristics of final pleochroic dyes. ##STR4##
That is, it is no exaggeration to say that development of a pleochroic dye necessary for realizing a guest-host type liquid crystal display having excellent characteristics depends upon selection of the kind, number and position of substituents on the fundamental perylene skeleton as well as selection of the fundamental skeleton.
In other words, mere selection of a fundamental skeleton of dye such as anthraquinone or perylene is usually insufficient, and a desired pleochroic dye can be obtained only by strictly specifying the kind, number, and position of substituents on the fundamental skeleton. The scope of such selection is not necessarily wide for a particular purpose.
The inventors have made investigations based on the above-mentioned situation and, as a result, the inventors have found that dyes with desired properties can be obtained by introducing specific substituents to specific positions of the perylene skeleton, thus having completed the present invention.